Problem: A jar contains $11$ red jelly beans, $7$ green jelly beans, and $3$ blue jelly beans. If a jelly bean is randomly chosen, what is the probability that it is blue?
There are $11 + 7 + 3 = 21$ jelly beans in the jar. There are $3$ blue jelly beans. The probability is $ \frac{3}{21} = \dfrac{1}{7}$.